Identification of Structural Factors in Iron Oxide Triggering Ortho–Para Hydrogen Conversion

The ortho–para hydrogen (o–p H2) conversion is an essential intensification process for the liquefaction, storage, and transportation of H2, which is highly desired for harnessing liquid hydrogen. Iron-based catalysts have been demonstrated as the most promising candidates for o–p H2 conversion because of their inexpensive and stable catalytic properties; however, the reactive active sites and catalytic mechanism remain hitherto unclear. In this contribution, a series of γ-Fe2O3 nanoparticles were prepared via a simple thermal treatment process and thoroughly characterized for exploring their structure–function relationship as o–p H2 conversion catalysts. A tremendous increased reaction rate constant for o–p H2 conversion was achieved using Fe2O3-2 as a catalyst, which is 2 orders of magnitude higher than that of the pristine γ-Fe2O3. Results from Mössbauer measurements demonstrated a positive correlation between the Fe3+ in the tetrahedral-site (A-site) and the o–p H2 conversion performance. Additionally, the higher saturation magnetization, pore volume, and specific surface area were also demonstrated to be critical for o–p H2 conversion. Our work provides an in-depth insight into the key structural factors in iron oxide triggering o–p H2 conversion, which shall pave the way toward the design of novel efficient catalysts for practical application.